1. Field of the Invention
This invention relates to drilling fluid compositions and to methods for drilling a subterranean wellbore or borehole. More particularly, this invention relates to compositions and methods for removing drill cuttings from boreholes and also for separating the cuttings from drilling fluids.
2. Description of Relevant Art
Rotary drilling methods employing drilling apparatus having a drill bit and drill stem have long been used to drill wellbores or boreholes in subterranean formations. Drilling fluids or muds are commonly circulated in the well during such drilling to serve a number of functions, including cooling and lubricating the drilling apparatus, counterbalancing the subterranean formation pressure encountered, and removing drill cuttings from the formation out of the wellbore. In removing drill cuttings from the well, drilling fluids suspend the cuttings and carry them to the surface for removal from the well.
Drilling deviated and horizontal wells have become increasingly common in the oil and gas industry. In drilling such wells, gravity causes deposits of drill cuttings, the sizes of which range from microns in diameter to that of common pebbles, and especially fines or smaller sized cuttings, to build up along the lower or bottom side of the wellbore. Such deposits are commonly called xe2x80x9ccuttings beds.xe2x80x9d As used herein, the term xe2x80x9cdeviatedxe2x80x9d with respect to wells shall be understood to include any well at sufficient angle or deviation off of vertical that cuttings beds tend to form during the drilling operation. xe2x80x9cDeviatedxe2x80x9d wells shall be understood to include without limitation xe2x80x9cangled,xe2x80x9d xe2x80x9chigh-angled,xe2x80x9d xe2x80x9coval,xe2x80x9d xe2x80x9ceccentric,xe2x80x9d xe2x80x9cdirectionalxe2x80x9d and xe2x80x9chorizontalxe2x80x9d wells, as those terms are commonly used in the oil and gas industry. The terms xe2x80x9cwell,xe2x80x9d xe2x80x9cwellborexe2x80x9d and xe2x80x9cboreholexe2x80x9d are synonymous as used herein.
The viscosity of a drilling fluid is commonly increased to enhance the fluid""s drill cuttings-transport capability. However, pumping high-viscosity fluids can be disadvantageous to the economics of oil well drilling by effecting high friction pressure, requiring higher horsepower pumping equipment and subsequent higher fuel expenditure. Higher drilling fluid viscosity is advantageous only in the annular space between drill pipe and borehole, where drill cuttings are located and from which they need to be removed. In other locations within the well during drilling, primarily inside the drillpipe and flow channels within the bit, lower viscosity is preferred for the drilling mud so as to minimize frictional pressure loss. The narrower flow channels inside the drillpipe and drill bit cause the drilling fluid to undergo a higher shear rate, which also increases frictional pressure loss. To counteract this undesirable occurrence, drilling fluids currently in common use are referred to as xe2x80x9cshear-thinningxe2x80x9d fluids because they have been designed to have a higher viscosity when at lower shear rate and lower viscosity in higher shear rate conditions. This serves, to some extent, to satisfy both the need for higher viscosity in the wellbore annulus and lower viscosity inside the drill pipe and drill bit. However, the current state of the art in drilling fluids design allows for only a limited degree of control of the variance in fluid viscosity between these various locations in the well being drilled.
Cleaning (i.e., removing drill cuttings from) a deviated well, particularly drilled at a high angle, can be difficult. Limited pump rate, limited drilling fluid density, eccentricity of the drill pipe, sharp build rates, and oval-shaped wellbores can all contribute to inadequate hole cleaning. In turn, inadequate hole cleaning can lead to cuttings beds build-up in the wellbore, because commonly used drilling fluids are often unable to sufficiently remove cuttings from such cuttings beds while circulating through the wellbore.
Buildup of cuttings beds can lead to undesirable friction and possibly to sticking of the drill string. Such buildup is especially a problem in Extended Reach Drilling, in which the majority of the length of the well is deviated from vertical by more than 40 degrees.
Well treatments or circulation of fluids specially formulated to remove these cuttings beds are periodically necessary to prevent buildup to the degree that the cuttings or fines interfere with the drilling apparatus or otherwise with the drilling operation. Two commonly used types of treatment fluids that have been applied with limited success are highly viscous fluids, having greater viscosity than the drilling fluids being used in the drilling operation, and lower viscosity fluids, having less viscosity than the drilling fluids being used in the drilling operations. Commonly, the drilling operation must be stopped while such treatment fluids are swept through the wellbore to remove the fines. It is desired, but difficult, to prevent intermixing of these treatment fluids with the drilling fluid. Such occurrences can be problematic in that they may alter the physical properties, such as density, of the drilling fluid.
A new method taught in U.S. Pat. No. 6,290,001, issued Sep. 18, 2001 to West et al., enables a sweep without stopping the drilling operation. In that method, a sweep material is added to the wellbore drilling fluid, either directly or in a carrier fluid compatible with the drilling fluid. The sweep material is circulated in the well, where it dislodges, suspends or pushes drill cuttings, especially fines and smaller sized cuttings deposited on the lower side of the wellbore or in cuttings beds, to the surface of the well. The sweep material is then removed from the drilling fluid, preferably by sieving or screening, so the drilling fluid may be returned to the wellbore without significant change in density. The sweep material comprises a weight material, such as barium sulfate, that has been ground and sieved to a specific grind size sufficiently small to be suspendable in the drilling fluid and generally harmless to the fluid pumping apparatus but sufficiently large to be screened out of the drilling fluid, preferably by the principal shale shaker for the drilling operation.
There continues to be a need, however, for more methods and materials for removing drill cuttings from wellbores.
The method of the present invention employs a drilling fluid whose viscosity increases after the fluid passes through the drill bit nozzles in the borehole and decreases after the fluid returns to the well surface. This viscosity change is effected by using a drilling fluid containing a polymer that can be caused to crosslink (which increases the fluid""s viscosity) downhole. The crosslinking can be reversed after the fluid returns to the well surface to facilitate ease of removal of drill cuttings and recycling of the drilling fluid.
Such delayed and reversible crosslinking may be effected in a number of ways. A preferred approach is to provide a drilling fluid comprising an aqueous base, a crosslinkable polymer, and a crosslinking agent. A crosslink activator encapsulated in an encapsulant is provided in the drilling fluid. The crosslink activator may be the crosslinking agent or it may be an agent that facilitates crosslinking of the polymer by the crosslinking agent, such as a pH adjusting compound. The encapsulant comprises a material or composition that can maintain its integrity and contain the crosslink activator apart from the polymer when introduced into the fluid before injection into the well but which breaks up or dissolves in the wellbore releasing the crosslink activator into the drilling fluid. The breaking up or dissolving of the encapsulant may be due to shearing caused by passing the fluid through the drill nozzles or may be due to increased temperature in the wellbore. Other suitable means for breaking up or dissolving of the encapsulant may alternatively be used. Once released into the drilling fluid, the crosslink activator can effect the crosslinking of the polymer. The drilling fluid containing the crosslinking and crosslinked polymer is circulated in the wellbore where it entrains drill cuttings.
When the drilling fluid, which contains drill cuttings, reaches the well surface, the crosslinking is reversed (which reduces the fluid""s viscosity). The drill cuttings are then removed from the fluid and additional encapsulated crosslinking activator is added back to the fluid (along with any other appropriate or needed additives, such as weighting agents to provide or maintain desired density, to complete the drilling fluid) for recirculation of the fluid in the wellbore.
An advantage of this method is that highly viscous fluids may be used for removing drill cuttings from the well without forcing such viscous fluids through the drill bit nozzles and hence without taxing pumping equipment. Further, such viscous fluids may be used as the drilling fluid, without altering the density of the drilling fluid, and without stopping the drilling for a sweep of the wellbore with viscous fluid to remove drill cuttings.
Another advantage of the method of the invention is that it allows flexibility during the drilling operation itself. The viscosity of the fluid may be adjusted as frequently as each cycle of drilling fluid in the wellbore. Although such frequency is not likely to be needed, it demonstrates the flexibility of the method. Thus, as the fluid rheology and other drilling conditions and subterranean formation characteristics (i.e., pore pressure, rock types, oil/gas/water saturation, etc.) are being monitored real-time during drilling, and if such formation characteristics and drilling conditions change, the fluid viscosity may be changed according to the method of the invention to quickly adapt to such changes in the formation. The fluid viscosity may be quickly changed by changing the amount or kind of crosslink activator being added back into the fluid at the well surface.